| What can we make of the complications that led the Large Blue butterfly (Maculinea arion) to extinction in Britain? |
|
| At first sight, nothing could seem less intelligent than the design of a flying insect. From an egg laid in or on a food supply, it hatches into a slow-moving eating machine that keeps outgrowing its skin, so that it has to molt every few days. At the moment of molting, it is extremely vulnerable to predators and parasites. Then, inexplicably, it stops moving and grows a hard shell, inside which it completely redesigns its body from square one, to emerge into a thing with wings that launches itself into hundreds of cubic miles of atmosphere in search of a mate, and a food plant, with nothing to guide it but a few stray molecules - pheromones and plant odors - blowing in the wind.
The fact is, however, that this is a very efficient system for spreading the genes of that species around the landscape, and for locating food plants that would take an Earth-bound caterpillar days to find by dint of much hard crawling. The proof is that there are more species of insect than any other class of animal, and their biomass outweighs the mammals, even though the latter include all the elephants on earth and close to a billion overweight humans as well.
OK, that complicated life cycle seems an intelligent creation in the end. But what can we make of the further complications that led the Large Blue butterfly (Maculinea arion) to extinction in Britain? It entrusts a critical stage in its life cycle to the tender care of a single species of red ant that is particularly finicky about where it nests.
The story goes like this: The Large Blue lays its eggs in the buds of thyme - the culinary herb that grows wild in Europe - in the tight-bud stage. If the butterfly is ready to lay its eggs before the buds appear, or not until after they have started to open, the brood is lost. The eggs hatch after one or two weeks, depending on the weather; warm weather speeds hatching. The young caterpillars feed on thyme flowers for about two weeks during late July and early August, then fall to the ground where they are "adopted" by red ants (Myrmica sabuleti) attracted by a sugary substance secreted from a dorsal gland. The ants carry the caterpillar back to their nest, where it then gorges on ant larvae. While hidden from its own predators, the caterpillar spends 10 months as a predator in the ant nest, and then pupates there. After three weeks pupation the butterfly emerges during the four weeks mid-June to mid-July.
M. sabuleti is a warmth-loving ant that thrives only in short, dry grassland on hot south-facing slopes that are heavily grazed. If the grass grows higher than 3-4 cm and shades the ground, cooling it, this ant dies out and other species of ant take over - ants that are not interested in providing free food and lodging for Large Blue caterpillars. Taller grass also crowds out thyme.
What happened in Britain was a constellation of events that conspired to spell disaster for the Large Blue. One was the increased use of chemical fertilizers that promote vigorous grass growth, which kills off small wild flowers such as thyme. Then, sheep were pulled off the land by a change in livestock farming. For a few years, rabbits spread and kept the grass short in habitats favored by the butterfly, but in the 1950s myxomatosis (a viral disease of rabbits) was introduced and eliminated them. Pastures also were previously burned over, which kept the grass short, but this is no longer done.
So here you have an insect that depends for its very existence on a fragile chain of circumstances that is easily broken by bad weather, changes in exposure to grazing due to human intervention and disease, loss of its unique food plant, and loss of its protector ant species. If I were to design such a silly system I'd at least choose the most abundant, hardy species of ant to host my caterpillars, and ensure that they could feed on other plants beside thyme, and at other stages than the bud. To me, the case of the Large Blue is conclusive disproof of the theory of intelligent design.
Jack Woodall is director of the Nucleus for the Investigation of Emerging Infectious Diseases in the Institute of Medical Biochemistry at Brazil's Federal University of Rio de Janeiro. |
